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versión On-line ISSN 1669-9637

Rev. Unión Mat. Argent. v.47 n.1 Bahía Blanca ene./jun. 2006

The special geometry of Euclidian supersymmetry: a survey

Vicente Cortés

Abstract: This is a survey about recent joint work with Christoph Mayer, Thomas Mohaupt and Frank Saueressig on the special geometry of Euclidian supersymmetry. It is based on the second of two lectures given at the II Workshop in Differential Geometry, La Falda, Córdoba, 2005.

Introduction

The purpose of this note is to present some geometric structures and constructions which arise in the study of supersymmetric field theories on a Euclidian rather than Minkowskian space-time, see [CMMS1, CMMS2].

The text is written for readers with a background in differential geometry. Nevertheless, a rough idea of supersymmetry and the restrictions it imposes on the geometry of the scalar manifold would be helpful for orientation. A short introduction to classical (rigid) supersymmetric field theories and to the notion of special geometry is provided in another contribution to this volume, based on the first of my two talks given at the II Workshop in Differential Geometry. Such theories are usually considered on the d-dimensional Minkowski space. The case is particularly important since our observed space-time is four-dimensional.

For various physical reasons, it is useful to consider also field theories on Euclidian space, even if our physical space-time metric is not positive definite. In quantum field theory, for instance, one needs to understand instantons, since they contribute to the Feynman path integral. Instantons are particular solutions of the Euler-Lagrange equations of a Euclidian counterpart of the underlying Minkowskian classical field theory.

For the Euclidian 4-space there exists an super-Poincaré algebra and Euclidian vector multiplets can be defined [CMMS1]. (There exists no super-Poincaré algebra on the Euclidian 4-space.) The special geometry of the scalar manifold in supersymmetric field theories with Euclidian vector multiplets was determined in [CMMS1] and named (affine) special para-Kähler geometry, since the role of the complex structure in special Kähler geometry is now played by a para-complex structure , . Special para-Kähler manifolds are discussed in section 1.

In section 2 we present several maps, which relate various special geometries associated to field theories in five, four and three dimensions. In particular, we discuss two purely geometric constructions of para-hyper-Kähler manifolds, which correspond to the dimensional reduction of a Euclidian or Minkowskian theory in 4 dimensions to a Euclidian theory in 3 dimensions [CMMS2].

By the lemma defines a system of local coordinates. Therefore, there exists a unique flat and torsion-free connection on for which and are affine functions.

Theorem 1. [CMMS1] Letbe a PKLI with induced dataThenis a special para-Kähler manifold.

Conversely, any simply connected special para-Kähler manifoldadmitsa PKLIwith induced dataMoreover, the PLKIis uniqueup to an element of

Proof of "". Let be a PKLI with induced data We have to show that is special para-Kähler. We know that is para-Kähler and that is flat and torsion-free. By the lemma, the symplectic form has constants coefficients with respect to the -affine coordinates Thus It remains to show that is symmetric. For a -parallel one-form and vector fields on we calculate:

Therefore, it is sufficient to prove that is closed for and . Let us check this, for example, for . The function is the real-part of the para-holomorphic function So . Since and are closed, this shows that is closed.

Corollary 1.Letbe a para-holomorphic function defined on a open setsatisfying the non-degeneracy condition

Then,

is a PKLI and hence defines a special para-Kähler manifold

Conversely, any special para-Kähler manifold is locally of this form.

2. Maps between special geometries from dimensional reduction

Dimensional reduction is a procedure for the construction of a field theory in space-time dimensions from one in dimensions.

In the context of special geometry of Euclidian supersymmetry it is natural to ask the following two questions:

Is it possible to construct supersymmetric field theories with vector multiplets on 4-dimensional Euclidian space from field theories on 5-dimensional Minkowski space?

Is it possible to construct Euclidian supersymmetric field theories in 3 dimensions out ofsupersymmetric field theories with vector multiplets in 4 dimensions?

The first question is given a detailed positive answers in [CMMS1], the second in [CMMS2]. We describe the corresponding geometrical constructions in the remaining two subsections.

2.1. Dimensional reduction from 5 to 4 dimensions. The allowed target geometry for the scalar fields in the relevant supersymmetric theories on 5-dimensional Minkowski space is called (affine) very special, see [CMMS1].

It is defined by a real cubic polynomial with non-degenerate Hessian on some domain

We found that dimensional reduction of such a Minkowskian theory over time yields a Euclidian supersymmetric theory with vector multiplets such that the target is special para-Kähler [CMMS1]. This means that we get a map:

which we call the para-r-map.

Theorem 2. [CMMS1] There exists a mapwhich associates a special para-Kählerstructure on the domainto any very special manifold,The special para-Kähler structure is defined by the para-holomorphic function

which satisfies

This is the para-version of the r-map:

introduced by B. de Wit and A. Van Proeyen in [DV] in the context of supergravity.

2.2. Dimensional reduction from 4 to 3 dimensions. We found two ways of constructing Euclidian supersymmetric field theories in 3 dimensions out of theories with vector multiplets in 4 dimensions [CMMS2].

One can start either with a Minkowskian theory and reduce over time or with a Euclidian theory. This gives us two maps:

which we call the para-c-maps. They are para-variants of the c-map, worked out by Cecotti, Ferrara and Girardello in [CFG].

Para-hyper-Kähler manifolds.

Definition 4.A para-hyper-Kähler manifold is a pseudo-Riemannian manifold withthree pairwise anticommuting parallel skew-symmetric endomorphism fields, such that A pseudo-Riemannian manifold is para-hyper-Kähler if and only if its holonomy group

Here stands for the standard symplectic structure of . In particular, the dimension of any para-hyper-Kähler manifold is divisible by 4.

The para-c-maps. Now I describe, for instance, the para-hyper-Kähler manifold associated to a special para-Kähler manifold via the para-c-map Let be the total space of the cotangent bundle and consider the decomposition , into horizontal and vertical subbundles with respect to the connection

This defines a canonical identification

With respect to the above identification, we define a pseudo-Riemannian metric on by

and two involutionsby

Theorem 3. [CMMS2] For any special para-Kähler manifold,is a para-hyper-Kähler manifold.

Conclusion. The maps between special geometries induced by dimensional reduction are summarized in the following diagram:

The diagram is essentially commutative:

Theorem 4. [CMMS2] For any very special manifoldthe para-hyper-Kähler manifoldsandare canonically isometric.